U.S. Pat. No. 2,684,331 (to Bauman) discloses a method for separating chromatographically from one another two or more substances having widely different ionization constants in which at least one of the substances undergoes considerable ionization in a dilute aqueous solution thereof. However, the method has not been used for separating sugars. The examples of U.S. Pat. No. 2,684,331 describe separation of salts from organic solvents, e.g. sodium chloride from formaldehyde. The method comprises an ion exchange resin having an ion identical with an ion of highly ionized solute. The ion exchange resin is either a cation exchange resin having an acidic form or an anion exchange resin having a basic form. The cation exchange resin contains sulphonic acid groups. The anion exchange resin contains quaternary ammonium groups.
U.S. Pat. No. 2,911,362 (to Wheaton) describes a method comprising a chromatographic separation process employing ion exchange resins for separating two or more water soluble organic compounds from one another in an aqueous medium in the absence of an ion exchange reaction, i.e. in the substantial absence of a chemical reaction involving an absorption of ions from the aqueous medium by the resin or the introduction of ions into the solution from the resin. According to said method the ion exchange resin can be either a cation exchange resin or an anion exchange resin. The cation exchange resin may contain either sulfonic acid groups or carboxylic acid groups. The anion exchange resin contains quaternay ammonium groups. However, the method has not been used for separating sugars.
Chromatographic separation has been used for recovering xylose from hydrolysates of natural materials such as birch wood, corn cobs and cotton seed hulls in a method described in U.S. Pat. No. 4,075,406 (to Melaja, et al.). The resin employed in the chromatographic separation is a strongly acid cation ex-changer, i.e. sulfonated polystyrene cross-linked with divinyl benzene. The use of a strongly acid cation exchanger for separating monosaccharides, e.g. xylose, from magnesium sulfite cooking liquor is also known from Finnish Patent Application No. 962 609. The chromatographic separation is carried out by using a simulated moving bed. However, the separation of certain monosaccharides by using strongly acid cation exchange resins has turned out to be difficult. For instance the separation of rhamnose from other carbohydrates with strongly acid cation exchange resins and strongly basic cation exchange resins has been possible by using solvents such as alcoholic solvents as eluants (see e.g. Samuelson O., Chromatography on ion exchange resins, J. Methods Carbohy. Chem. 6 (1972) 65–75). In the described system anhydro sugars, such as rhamnose, have a shorter retention time than most of the aldoses and ketoses. Water would be a preferred eluant, but the use of water has not, however, been described in this connection. The problem when using water is that the various monosaccharides, such as rhamnose, xylose and arabinose, have almost similar retention times, whereby the fractions will overlap.
The separation of carbohydrates, especially xylose by strongly acid cation exchangers has been practiced industrially but is complicated. The method presented in U.S. Pat. No. 5,998,607 (to Heikkila, et al.) has been used especially for separating xylose from magnesium spent liquor. The problem has been the unsufficient separation of xylose and xylonic acid and there is no suggestion of the use of a weakly acid cation exchange resin possibly giving a benefit for solving the problem. In the disclosed method the separation requires two steps. In the first step the cation exchange resin is used preferably in alkaline earth form, more preferably in Mg2+ form and in the second step the cation exchange resin is preferably in alkali-metal form (e.g. sodium). However, the separation of monosaccharides has also been found to be unsatisfactory since all the other sugars elute at almost similar retention time with xylose. The pH used in the process low. The resin in a divalent form seemed to separate the xylose more effectively than the resin in a monovalent form.
Anion exchange resins have been used for separating fructose from glucose. Y. Takasaki (Agr. Biol. Chem. 36 (1972) pages 2575–77) and B. Lindberg et al. (Carbohyd. Res. 5 (1967), pages 286–291) describe the use of an anion exchanger in bisulfite form for the separation of sugars. However, the use of anion exchange resins does not result in good xylose separation because xylose is overlapped by other sugars.
U.S. Pat. No. 4,358,322 (to Neuzil, et al.) discloses a process for separating fructose from a feed mixture comprising fructose and glucose. The process comprises contacting the mixture with an adsorbent comprising aluminosilicate or zeolite. The adsorbent contains one or more selected cations at exchangable cation sites. The cations are selected from the group consisting of sodium, barium and strontium. The cationic pairs used in the cationic sites are selected from the group consisting of barium and potassium and barium and strontium.
U.S. Pat. No. 5,084,104 (to Heikkila, et al.) discloses a method for the separation of xylose from a pentose-rich solution, e.g. birch wood. A chromatographic column which comprises a strongly basic anion exchange resin is used. The anion exchange resin is in sulfate form. Using this method xylose is retarded most strongly and the other monosaccharides are eluted faster.
A method for preparing of L-arabinose is known from the publication WO 99/57326 where the process is characterized by contacting plant fibers with an acid to hydrolyze the fibers under such conditions that the L-arabinose ingredients contained in the plant fibers are selectively obtained. U.S. Pat. No. 4,880,919 (to Kulprathipanja) discloses a process for separating arabinose from mixtures of monosaccharides containing arabinose and other aldopentoses and aldohexoses by adsorption on sulfonated polystyrene divinyl benzene crosslinked ion exchange resins exchanged with Ca2+ and NH4+ ions and desorpting the adsorbate with water. A process for producing crystalline L-arabinose is known from U.S. Pat. No. 4,816,078 (to Schiweck, et al.).
The preparation of arabinose is also known from U.S. Pat. No. 4,664,718 (to Chang). In the described method, arabinose is separated from a mono-saccharide mixture containing also other aldopentoses and aldohexoses. The feed is contacted with with a calcium-Y-type or calcium-X-type zeolite and arabinose is adsorbed selectively. The desorption is conducted with water or ethanol.
Publication DE 3 545 107 describes a method for the preparation of rhamnose from arabic gum. A strongly acid cation exchange resin is used for the separation of sugars and rhamnose by adsorption with activated charcoal. Arabinose is also separated by this method.
Barker, S. A. et al (Carbohydrate Research, 26 (1973) 55–64) have described the use of poly(4-vinylbenzeneboronic acid) resins in the fractionation and interconversion of carbohydrates. In the method water is used as an eluant. The best yield of fructose was received when the pH was high. The resins have also been used to displace the pseudo equilibrium established in aqueous alkali between D-glucose, D-fructiose and D-mannose to yield D-fructose.
CA Patent No. 1 249 812 discloses a multistep process for the separation of sugars and lignosulphonates fron sulphite spent liquor. The process comprises the steps of (a) introducing sulphite spent liquor having a certain pH into a chromatographic column containing a resin in metal salt form, (b) eluting the column with water to obtain a substiantially sugar-free lignosulphonate-rich fraction and a sugar-rich fraction, (c) collecting the sugar-rich fraction for further purification, (d) adjusting the pH of the fraction to a certain level and introducing it to a second column containing a resin in monovalent metal salt form, and (e) eluting the sugar-rich material from the second column to obtain a sugar-rich fraction and a lignosulphonate-rich fraction. The process of said CA patent does not include the use of a weakly acid cation exchange resin for chromatographic separation.
A process for crystallizing xylose is known from Finnish Patent 97 625. In the process xylose is recovered by crystallization from the solutions in which the xylose purity is relatively low. Especially the process concerns recovering xylose from biomass derived solutions.
When xylose is prepared by hydrolysing biomass derived xylose rich hemicellulose the mixture contains among xylose also glucose, galactose, rhamnose, mannose and arabinose. It also may contain acetic acid and uronic acids such as galacturonic acid and glucuronic acid. The hydrolysing acid and the uronic acid are generally easily removed by ion exclusion. However, it has been difficult to fractionate monosaccharide mixtures to their components.
Surprisingly it has been found that rhamnose and, if desired, arabinose can be effectively separated from carbohydrate streams by using weakly acid cation exchange resins. The order of elution seems to be, besides other factors, affected strongly by the hydrophobic/hydrophilic interaction between the carbohydrate and the resin. If the resin is in hydrophilic form, the most hydrophobic carbohydrate seems to elute first and the most hydrophilic last. For instance, the resin in H+ form seems to be less hydrophilic than the resin in Na+ form. The different elution order of components in a chromatographic column using a weakly acid cation exchange resin can be effectively used in the method of the present invention comprising a multistep process.